Downmixing device and method

ABSTRACT

A downmixing device includes: a matrix conversion unit configured to perform a matrix operation for an input signal; a rotation correction unit configured to rotate an output signal of the matrix conversion unit; a spatial information extraction unit configured to extract spatial information from the output signal of the rotation correction unit; and an error calculation unit configured to calculate an error amount of the matrix operation result for the input signal by performing a matrix operation for the output signal of the rotation correction unit and the spatial information extracted by the spatial information extraction unit using a matrix that is inverse to the matrix used for the matrix operation by the matrix conversion unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-78570, filed on Mar. 30, 2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein relate to a downmixing device and a downmixing method.

BACKGROUND

Conventionally, downmix technologies are known that convert an audio signal of a plurality of channels into an audio signal of the fewer number of channels. As one of the downmix technologies, there is a predictive downmix technology. As one encoding method that uses the predictive downmix technology, there is a Moving Picture Experts Group (MPEG) surround method of International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC). In the MPEG surround method, two stages of downmixing processing are performed when an input signal of six channels that is generally called 5.1 channels is downmixed to two channel signals.

For example, among six-channel signals, two-channel signals are downmixed to a one-channel signal respectively to obtain three channel signals in the first stage of downmixing processing. In the second stage of the downmixing processing, a matrix conversion, for example, by the following expression (1) is applied, for example, to the signal of three channels, L_(in), R_(in), and C_(in) that are obtained in the first stage of the downmixing processing. In the expression (1), D indicates a downmix matrix, and represented, for example, by the second expression (2).

$\begin{matrix} {{Expression}\mspace{14mu} 1} & \; \\ {\begin{bmatrix} l_{0} \\ r_{0} \\ {\hat{c}}_{0} \end{bmatrix} = {D\begin{bmatrix} L_{in} \\ R_{in} \\ C_{in} \end{bmatrix}}} & (1) \\ {{Expression}\mspace{14mu} 2} & \; \\ {D = \begin{bmatrix} 1 & 0 & {\frac{1}{2}\sqrt{2}} \\ 0 & 1 & {\frac{1}{2}\sqrt{2}} \\ 1 & 1 & {{- \frac{1}{2}}\sqrt{2}} \end{bmatrix}} & (2) \end{matrix}$

The vector ĉ₀ obtained by the expression (1) is decomposed into a linear sum of two vectors, l₀ and r₀ as represented by the following expression (3). In the present disclosure, ĉ indicates that “̂” is placed over the “c.” In the expression (3), k₁ and k₂ are coefficients. The predicted signal c₀ is represented by the expression (4), when Channel Prediction Coefficients (CPC) that are substantially the closest to the k₁ is c₁ and k₂ is c₂.

Expression 3

{circle around (c)} ₀ =k ₁ ×l ₀ +k ₂ ×r ₀  (3)

Expression 4

c ₀ =c ₁ ×l ₀ +c ₂ ×r ₀  (4)

Japanese Laid-open Patent Publication No. 2008-517337 (WO2006/048203: May 11, 2006) discusses a downmix technology in which a scaling correction is applied to a downmix signal based on an energy difference between an input signal and an upmix signal to compensate an energy loss caused when a signal of a plurality of channels are generated from the downmix signal. Moreover, Japanese Laid-open Patent Publication No. 2008-536184 (WO2006/108573: Oct. 19, 2006) discusses an encoding technology in which a rotation matrix inverse to a rotation matrix to be used for upmixing processing is applied to left and right channel signals beforehand when executing downmixing processing in order to apply the rotation matrix to be used for upmixing processing to the downmix signal and the residual signal when executing upmixing processing.

SUMMARY

A downmixing device includes: a matrix conversion unit configured to perform a matrix operation for an input signal; a rotation correction unit configured to rotate an output signal of the matrix conversion unit; a spatial information extraction unit configured to extract spatial information from the output signal of the rotation correction unit; and an error calculation unit configured to calculate an error amount of the matrix operation result for the input signal by performing a matrix operation for the output signal of the rotation correction unit and the spatial information extracted by the spatial information extraction unit using a matrix that is inverse to the matrix used for the matrix operation by the matrix conversion unit, wherein the rotation correction unit determines a final rotation result based on the error amount calculated by the error calculation unit; and the spatial information extraction unit determines final spatial information based on the error amount calculated by the error calculation unit.

The object and advantages of the invention will be realized and attained by at least the features, elements and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a downmixing device according to a first embodiment;

FIG. 2 is a flow chart illustrating a down mixing method according to the first embodiment;

FIG. 3 is a characteristic chart illustrating a result of comparison between the first embodiment and a comparison example;

FIG. 4 is a block diagram illustrating a downmixing device according to a second embodiment;

FIG. 5 illustrates a time-frequency conversion in the downmixing device according to the second embodiment;

FIG. 6 is an example of MPEG-2 ADTS format; and

FIG. 7 is a flow chart illustrating a downmixing method according to the second embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, issues related to the present disclosure will be pointed out, and embodiments of the present disclosure will be described.

In the above-described background, when vectors of input signals L_(in) and R_(in) are substantially the same, vectors of l₀ and r₀ obtained by a matrix conversion become substantially the same (refer to expressions 1 and 2). In this case, the vector c{circle around ( )}₀ may not be completely reproduced by a linear sum of the two vectors l₀ and r₀, (refer to the expression (3)) and a phase of a predicted signal c₀ becomes the same phase as the phases of the l₀ and r₀.

At a decoder side, for example, an output signal of the three channels, L_(out), R_(out), and C_(out) are generated by applying an inverse matrix conversion to the l₀, r₀, c₁ and c₂ in the upmixing processing. At that time, when phases of the l₀, r₀, and c₀ are substantially the same, phases of the output signals of L_(out), R_(out), and C_(out) become substantially the same phases as well. Thus, the original input signals of L_(in), R_(in), and C_(in) at the encoder side may not be reproduced at the decoder side with high accuracy. In other words, there is a disadvantage in that sound quality is degraded through the matrix conversion in the downmixing processing and the inverse matrix conversion in the upmixing processing.

Hereinafter, embodiments of the downmixing device and the downmixing method will be described in detail by referring to the accompanying drawings. The downmixing device and the downmixing method suppress degradation of sound reproduced at a decoder side by applying a rotation correction to a downmix signal obtained from an input signal based on an error amount of an upmix signal obtained from the downmix signal for the input signal.

First Embodiment

Description of a Downmixing Device

FIG. 1 is a block diagram illustrating a downmixing device according to the first embodiment. As illustrated in FIG. 1, the downmixing device includes a matrix conversion unit 1, a rotation correction unit 2, a spatial information extraction unit 3, and an error calculation unit 4. The matrix conversion unit 1 performs a matrix operation for input signals, L_(in), R_(in), and C_(in). The matrix conversion unit 1 may perform a matrix operation indicated by the above-described expressions (1) and (2). According to the matrix operation, vectors of the two channels, l₀ and r₀, and a vector of a signal to be predicted c{circle around ( )}₀ are obtained.

The rotation correction unit 2 performs a rotation operation for the l₀ and r₀ that are output from the matrix conversion unit 1. The rotation correction unit 2 may perform a matrix operation indicated by the following expressions (5) and (6). In the expression (5), θ_(l) is a rotation angle of l₀, while θ_(r) is a rotation angle of r₀. Vectors l₀′ and r₀′ are obtained by rotating the vectors of the two channels, l₀ and r₀ through the matrix operation. The rotation correction unit 2 may perform a rotation operation for the l₀ and r₀ typically when vectors of the l₀ and r₀ are substantially the same.

$\begin{matrix} {{Expression}\mspace{14mu} 5} & \; \\ {\begin{bmatrix} l_{0}^{\prime} \\ r_{0}^{\prime} \end{bmatrix} = {\begin{bmatrix} ^{{\theta}_{1}} & 0 \\ 0 & ^{{\theta}_{r}} \end{bmatrix}\begin{bmatrix} l_{0} \\ r_{0} \end{bmatrix}}} & (5) \\ {{Expression}\mspace{14mu} 6} & \; \\ {^{\theta} = {{\cos \; \theta} + {{ \cdot \sin}\; \theta}}} & (6) \end{matrix}$

The rotation correction unit 2 determines l₀′ and r₀′ that become a final rotation result based on an error amount E calculated by the error calculation unit 4. For example, the rotation correction unit 2 may determine l₀′ and r₀′ when the error amount E is substantially the minimum as a final rotation result. The l₀′ and r₀′ that are determined as the final rotation result becomes a part of an output signal of the downmixing device illustrated in FIG. 1.

The spatial information extraction unit 3 extracts spatial information based on the output signals, l₀′ and r₀′ of the rotation correction unit 2. The spatial information extraction unit 3 may decompose the vector to be predicted c{circle around ( )}₀ obtained by the matrix conversion unit 1 into a linear sum of two vectors l₀′ and r₀′. The spatial information extraction unit 3 may obtain channel predictive parameters c₁ and c₂ as spatial information that are substantially closest to the coefficient k₁ of the l₀′ and the coefficient k₂ of r₀′. The channel predictive parameters c₁ and c₂ may be provided by a table. A vector c₀′ of a predictive signal may be obtained by the expression (7) below by using two vectors l₀′ and r₀′ corrected by the rotation correction unit 2 and the channel predictive parameters c₁ and c₂.

Expression 7

c ₀ ′=c ₁ ×l ₀ ′+c ₂ ×r ₀′  (7)

The spatial information extraction unit 3 determines channel predictive parameters, c₁ and c₂ that become final spatial information based on an error amount E calculated by the error calculation unit 4. For example, the spatial information extraction unit 3 may determine c₁ and c₂ when the error amount E is substantially the minimum as final spatial information. The c₁ and c₂ that are determined as the final spatial information become a part of an output signal of the downmixing device illustrated in FIG. 1.

The error calculation unit 4 performs a matrix operation for the l₀′ and r₀′ that are corrected by the rotation correction unit 2 and the c₁ and c₂ that are extracted by the spatial information extraction unit 3. The error calculation unit 4 may perform a matrix operation by using an inverse matrix of the matrix, for example, used in the matrix operation by the matrix conversion unit 1. In other words, the error calculation unit 4 may perform a matrix operation represented, for example, by the expressions (8) and (9). In the expression (8), the D⁻¹ is, for example, an inverse matrix of the downmix matrix represented by the above-described expression (2). The c₀′ is obtained by the expression (7). Through the matrix operation, upmix vectors of three channels, L_(out), R_(out), and C_(out) are obtained.

$\begin{matrix} {{Expression}\mspace{14mu} 8} & \; \\ {\begin{bmatrix} L_{out} \\ R_{out} \\ C_{out} \end{bmatrix} = {D^{- 1}\begin{bmatrix} l_{0}^{\prime} \\ r_{0}^{\prime} \\ c_{0}^{\prime} \end{bmatrix}}} & (8) \\ {{Expression}\mspace{14mu} 9} & \; \\ {D^{- 1} = {\frac{1}{3}\begin{bmatrix} 2 & {- 1} & 1 \\ {- 1} & 2 & 1 \\ \sqrt{2} & \sqrt{2} & {- \sqrt{2}} \end{bmatrix}}} & (9) \end{matrix}$

The error calculation unit 4 calculates error amounts of the L_(out), R_(out), and C_(out) for the input signals, L_(in), R_(in), and C_(in). The L_(out), R_(out), and C_(out) are upmix signals for the input signals L_(in), R_(in), and C_(in). The error calculation unit 4 may calculate error power between the input signals and upmix signals for each of the three channels respectively as an error amount E, for example, as represented in the expression (10).

Expression 10

E=|L _(out) −L _(in)|² +R _(out) −R _(in)|² +|C _(out) −C _(in)|²  (10)

Description of the Downmixing Method

FIG. 2 is a flow chart illustrating a downmixing method according to the first embodiment. As illustrated in FIG. 2, when the downmixing processing starts, the matrix conversion unit 1 performs a matrix operation for the input signals L_(in), R_(in), and C_(in) (Operation S1). Through the matrix operation, l₀, r₀, and c{circle around ( )}₀ are obtained. Processing described below may be performed typically when vectors of the l₀ and r₀ are the same.

A variable “min” is provided and is set to MAX (substantially the maximum value) by the rotation correction unit 2 (Operation S2). The MAX (substantially the maximum value) is provided as an initial value for the variable “min.” The variable “min” is retained, for example, in a buffer. A rotation angle θ_(l) of the l₀ is set as an initial value by the rotation correction unit 2 (Operation S3). A rotation angle θ_(r) of the r₀ is set as an initial value by the rotation correction unit 2 (Operation S4). For example, initial values for the θ_(l) and the θ_(r) may be 0. The rotation correction unit 2 rotates the l₀ and r₀ by the set angles (Operation S5). As a result of the rotations, corrected vectors, l₀′ and r₀′ are obtained.

The spatial information extraction unit 3 extracts spatial information based on the l₀′ and r₀′ (Operation S6). Accordingly, channel predictive parameters, c₁ and c₂ are obtained by extracting the spatial information.

The error calculation unit 4 calculates c₀′ by using the l₀′, r₀′, c₁, and c₂. A matrix operation that is inverse to the matrix operation in the Operation S1 is applied to the c₀′, l₀′, and r₀′. Upmix signals L_(out), R_(out), and C_(out) are obtained by the matrix operation. The error calculation unit 4 calculates an error amount E of upmix signals L_(out), R_(out), and C_(out) for the input signals L_(in), R_(in), and C_(in) (Operation S7).

The error calculation unit 4 compares the error amount E obtained at Operation S7 with the variable min (Operation S8). When the error amount E is smaller than the variable min (Operation S8: Yes), the variable min is updated to the error amount E obtained at Operation S7. Moreover, the l_(0′) and r_(0′), obtained at Operation S5 and the c₁ and c₂ obtained at Operation S6 are retained, for example, in a buffer (Operation S9). When the error amount E is not smaller than the variable min (Operation S8: No), the variable min is not updated. Moreover, the l₀′, r₀′, c₁, and, c₂ may be or may not be retained (Operation S9).

The rotation correction unit 2 adds a Δ θ_(r) to the rotation angle θ_(r) and updates the rotation angle θ_(r). The θ_(r) may be, for example, π/180 (Operation S10). The updated rotation angle θ_(r) is compared with a rotation end angle θ_(rMAX) (Operation S11). The rotation end angle θ_(IMAX) may be 2π. When the rotation angle θ_(r) is smaller than the rotation end angle θ_(rMAX) (Operation S11: Yes), Operations S5 to S10 are repeated. When the updated rotation angle θ_(r) is not smaller than the rotation end angle θ_(rMAX) (Operation S11: No), Operations S5 to S10 are not repeated. The rotation correction unit 2 adds a Δ θ_(l) to the rotation angle θ_(l) and updates the rotation angle θ_(l). The θ_(l) may be, for example, π/180 (Operation S12). The updated rotation angle θ_(l) is compared with a rotation end angle θ_(IMAX) (Operation S13). The rotation end angle θ_(IMAX) may be 2π. When the rotation angle θ_(l) is smaller than the rotation end angle θ_(IMAX) (Operation S13: Yes), Operations S4 to S12 are repeated. When the rotation angle θ_(l) is not smaller than the rotation end angle θ_(IMAX) (Operation S13: No), Operations S4 to S12 are not repeated.

When processing from Operations S3 to S13 are completed for all of the rotation angles θ_(l) and θ_(r) in a range that is set, the series of the downmixing processing is completed. At this time, the l₀′, r₀′, c₁, and, c₂ when the error amount is substantially the minimum are retained, for example, in a buffer. In other words, the l₀′, r₀′, c₁, and, c₂ when the error amount is substantially the minimum are obtained. The downmixing device outputs the l₀′, r₀′, c₁, and, c₂ when the error amount is substantially the minimum.

Comparison of Error Amounts E

FIG. 3 is a characteristic chart illustrating a result of a comparison between the first embodiment and a comparison example. In FIG. 3, the vertical axis indicates an error amount E, while the horizontal axis indicates an angle “α.” The angle “α” is an angle between a vector of the input signal C_(in) and a vector of the L_(in) (R_(in)) where the vectors of the input signal L_(in) and R_(in) are assumed to be substantially the same. The graph for the first embodiment indicates a simulation result of the error amount E when the rotation correction unit 2 applies a rotation correction to the l₀′ and r₀′ that are output by the matrix conversion unit 1. The graph for the comparison example indicates a simulation result of the error amount E when the rotation correction unit 2 does not apply a rotation correction to the l₀ and r₀ that are output by the matrix conversion unit 1. As may be obvious from FIG. 3, the error amount E of the first embodiment is smaller than that of the comparison example.

According to the first embodiment, when the vectors of the input signals L_(in) and R_(in) are substantially the same, downmix signals l₀′ and r₀′ and channel predictive parameters, c₁ and c₂ when an error amount E of an upmix signal for the input signal becomes substantially the minimum are obtained. The downmixing device outputs values obtained by encoding the downmix signals l₀′ and r₀′ and channel predictive parameters, c₁ and c₂ when the error amount E becomes substantially the minimum to the decoder side. Accordingly, the input signal to the downmixing device may be reproduced with high accuracy when decoded at the decoder side and upmixing processing is applied based on the downmix signals l₀′ and r₀′ and channel predictive parameters, c₁ and c₂. In other words, degradation of sound quality may be suppressed when sound in which the vectors of the input signals L_(in) and R_(in) that are input to the downmixing device are substantially the same is reproduced at the decoding side.

Second Embodiment

The second embodiment uses the downmixing device according to the first embodiment as an MPEG Surround (MPS) encoder. MPS decoder and MPS decoding technologies are specified in International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 23003-1. The MPS encoder converts an input signal to a signal decodable by the specified MPS decoder. The downmixing device according to the first embodiment may be applied to other encoding technologies as well.

Description of the Downmixing Device

FIG. 4 is a block diagram illustrating a downmixing device according to the second embodiment. As illustrated in FIG. 4, the downmixing device includes a time-frequency conversion unit 11, a first Reverse one to two (R-OTT) unit 12, a second R-OTT unit 13, a third R-OTT unit 14, a Reverse two to three (R-TTT) unit 15, a frequency-time conversion unit 16, an Advanced Audio Coding (MC) unit 17, and a multiplexing unit 18. Functions of each of the components are achieved by executing an encoding process, for example, by a processor. In FIG. 4, a signal with “(t)” such as “L (t)” indicates that is a time domain signal.

The time-frequency conversion unit 11 converts time domain multi-channel signals that are input to the MPS encoder into frequency domain signals. In a 5.1 channel surround system, multi-channel signals are, for example, a left front signal L, a left side signal SL, a right front signal R, a right side signal SR, a center signal C, and a low-frequency band signal, Low Frequency Enhancement (LFE).

For the time-frequency conversion unit 11, for example, a complex type Quadrature Mirror Filter (QMF) bank indicated in the expression 11 may be used. FIG. 5 illustrates frequency conversions of an L channel signal. A case is illustrated in which the number of samples for the frequency axis is 64, and the number of samples for the time axis is 128. In FIG. 5, L (k, n) 21 is a sample of a frequency band “k” at time “n.” The same applies to signals of respective channels, the SL, R, SR, C and LFE.

$\begin{matrix} {{Expression}\mspace{14mu} 11} & \; \\ {{{{{QMF}\lbrack k\rbrack}\lbrack n\rbrack} = {\exp \left\lbrack {j\frac{\pi}{128}\left( {k + 0.5} \right)\left( {{2n} - 1} \right)} \right\rbrack}},{0 \leq k < 64},\mspace{14mu} {0 \leq n < 128}} & (11) \end{matrix}$

The R-OTT units 12, 13, and 14 downmix two-channel signals into one-channel signal respectively. The first R-OTT unit 12 generates a downmix signal L_(in) obtained by downmixing a frequency signal L of the L channel and a frequency signal SL of the SL channel. The first R-OTT unit 12 generates spatial information based on the frequency signal L of the L channel and the frequency signal SL of the SL channel. Spatial information to be generated is Channel Level Difference (CLD) that is a difference of levels between the downmixed two channels and an Inter-channel Coherence (ICC) that is an interrelation of the downmixed two channels. The second R-OTT unit 13 generates, in the same manner as the first R-OTT unit 12, a downmix signal R_(in), and spatial information (CLD and ICC) for the frequency signal R of the R channel and a frequency signal SR of the SR channel. The third R-OTT unit 14 generates, in the same manner as the first R-OTT unit 12, a downmix signal c_(in), and spatial information (CLD and ICC) for the frequency signal C of the C channel and a frequency signal LFE of the LFE channel.

Calculations by the first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 will be collectively described. The first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 may calculate a downmix signal M by the expression (12). The x₁ and x₂ in the expression (12), are signals of two channels to be downmixed. The first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 may calculate a difference of levels between channels, CLD by the expression (13). The first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 may calculate an Inter-channel Coherence (ICC) that is an interrelation of the channels by the expression (14).

$\begin{matrix} {{Expression}\mspace{14mu} 12} & \; \\ {M = {x_{1} + x_{2}}} & (12) \\ {{Expression}\mspace{14mu} 13} & \; \\ {{CLD} = {10\log \; 10\left( \frac{\sum\limits_{n}{\sum\limits_{k}{x_{1}^{n,k}x_{1}^{n,k^{*}}}}}{\sum\limits_{n}{\sum\limits_{k}{x_{2}^{n,k}x_{2}^{n,k^{*}}}}} \right)}} & (13) \\ {{Expression}\mspace{14mu} 14} & \; \\ {{ICC} = {{Re}\left( \frac{\sum\limits_{n}{\sum\limits_{k}{x_{1}^{n,k}x_{2}^{n,k^{*}}}}}{\sqrt{\sum\limits_{n}{\sum\limits_{k}{x_{1}^{n,k}x_{1}^{n,k^{*}}{\sum\limits_{n}{\sum\limits_{k}{x_{2}^{n,k}x_{2}^{n,k^{*}}}}}}}}} \right)}} & (14) \end{matrix}$

The R-TTT unit 15 downmixes three-channel signals into two-channel signals. The R-TTT unit 15 outputs the l₀′ and r₀′ and channel predictive parameters, c₁ and c₂ based on the downmix signals L_(in), R_(in), and C_(in) that are output from the three R-OTT units 12, 13, and 14 respectively. The R-TTT unit 15 includes a downmixing device according to the first embodiment, for example, as illustrated in FIG. 1. The R-TTT unit 15 will not be described in detail because that is substantially the same as that described in the first embodiment.

The frequency-time conversion unit 16 converts the l₀′ and r₀′ that are output signals of the R-TTT unit 15 into time domain signals. For the frequency-time conversion unit 16, for example, a complex type Quadrature Mirror Filter (QMF) bank represented in the expression (15) may be used.

$\begin{matrix} {{Expression}\mspace{14mu} 15} & \; \\ {{{{{IQMF}\lbrack k\rbrack}\lbrack n\rbrack} = {\frac{1}{64}{\exp\left( {j\frac{\pi}{64}\left( {k + \frac{1}{2}} \right)\left( {{2n} - 127} \right)} \right)}}},{0 \leq k < 32},\mspace{14mu} {0 \leq n < 32}} & (15) \end{matrix}$

The AAC encode unit 17 generates AAC data and an AAC parameter by encoding the l₀′ and r₀′ that are converted into time domain signals. For an encoding technology of the AAC encode unit 17, for example, a technology discussed in the Japanese Laid-open Patent Publication No. 2007-183528 may be used.

The multiplexing unit 18 generates output data obtained by multiplexing the CLD that is a difference of levels between channels, the ICC that is a correlation between channels, the channel predictive parameter c₁, the channel predictive parameter c₂, the AAC data and the MC parameter. For example, an MPEG-2 Audio Data Transport Stream (ADTS) format may be considered as an output data format. FIG. 6 illustrates an example of the MPEG-2 ADTS format. Data 31 with the ADTS format includes an ADTS header field 32, an MC data field 33, and a fill element field 34. The fill element field 34 includes an MPEG surround data field 35. MC data generated by the MC encode unit 17 is stored in the MC data field 33. Spatial information (CLD, ICC, c₁ and c₂) is stored in the MPEG surround data field 35.

Description of the Downmixing Method

FIG. 7 is a flow chart illustrating a downmixing method according to the second embodiment. As illustrated in FIG. 7, when downmixing processing starts, the time-frequency conversion unit 11 converts time domain multi-channel signals that are input to the MPS encoder into frequency domain signals (Operation S14). Operations S15 to S24 described below will be executed for each of the sample L (k, n) of the frequency band k at time n.

For a frequency band k at time n, 0 is set (Operation S15). For time n, 0 is set (Operation S16). In other words, processing is executed for multi-channel signals of frequency band 0 at time 0. The first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 calculate downmix signals L_(in), R_(in) and C_(in) for each channel signal of the frequency band 0. Moreover, the first R-OTT unit 12, the second R-OTT unit 13, and the third R-OTT unit 14 calculate the CLD that is a difference of levels between channels and the ICC that is a correlation between channels (Operation S17).

The R-TTT unit 15 calculates l₀′ and r₀′ after applying a rotation correction from the L_(in), R_(in) and C_(in). Moreover, the R-TTT unit 15 calculates channel predictive parameters, c₁ and c₂ (Operation S18). The processing procedure at Operation S18 will not be described in detail because it is substantially the same as, for example, the downmixing method according to the first embodiment illustrated in FIG. 2.

The frequency-time conversion unit 16 converts l₀′ and r₀′ into time domain signal (Operation S19). The MC encode unit 17 encodes (MC encode) the l₀′ and r₀′ that are converted into the time domain signal by applying an MC encoding technology to generate MC data and an MC parameter (Operation S20).

The time n is incremented for +1 and updated (Operation S21). The updated time n is compared with a substantially maximum value n_(max) (Operation S22). When the time n is smaller than the substantially maximum value n_(max) (Operation S22: Yes), Operations S17 to S21 are repeated. When the time n is not smaller than the substantially maximum value n_(max) (Operation S22: No), Operations S17 to S21 are not repeated.

The frequency k is incremented for +1 and updated (Operation S23). The updated frequency k is compared with a substantially maximum value k_(max) (Operation S24). When the frequency k is smaller than the substantially maximum value k_(max) (Operation S24: Yes), Operations S16 to S23 are repeated. When the frequency k is not smaller than the substantially maximum value k_(max) (Operation S24: No), Operations S16 to S23 are not repeated. When the MC encoding at Operation S20 for all combinations of samples for time n and frequency band k are completed, the multiplexing unit 18 multiplexes the CLD, ICC, c₁, c₂, MC data and MC parameter (Operation S25). The series of downmixing processing is completed.

According to the second embodiment, the downmixing device that is substantially the same as that of the first embodiment is provided. Thus, substantially the same effect as that of the first embodiment is achieved for the MPS encoder.

All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present invention(s) has(have) been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

1. A downmixing device comprising: a matrix conversion unit configured to perform a matrix operation for an input signal; a rotation correction unit configured to rotate an output signal of the matrix conversion unit; a spatial information extraction unit configured to extract spatial information from the output signal of the rotation correction unit; and an error calculation unit configured to calculate an error amount of the matrix operation result for the input signal by performing a matrix operation for the output signal of the rotation correction unit and the spatial information extracted by the spatial information extraction unit using a matrix that is inverse to the matrix used for the matrix operation by the matrix conversion unit, wherein the rotation correction unit determines a final rotation result based on the error amount calculated by the error calculation unit; and the spatial information extraction unit determines final spatial information based on the error amount calculated by the error calculation unit.
 2. The downmixing device according to claim 1, wherein the spatial information extraction unit calculates, as the spatial information, a coefficient for each vector when a signal to be predicted among output signals of the matrix conversion unit is decomposed into vectors of the output signal of the rotation correction unit.
 3. The downmixing device according to claim 1, wherein the rotation correction unit compares error amounts calculated by the error calculation unit while changing a rotation angle for the output signal of the matrix conversion unit to determine a rotation result when the error amount becomes substantially the minimum as a final output signal.
 4. The downmixing device according to claim 1, wherein the spatial information extraction unit determines spatial information that corresponds to a rotation result when an error amount calculated by the error amount calculation unit becomes substantially the minimum as final spatial information.
 5. The downmixing device according to claim 1, wherein the rotation correction unit determines a rotation result when an error amount calculated by the error calculation unit becomes substantially the minimum for each frequency band of the input signal; and the spatial information extraction unit determines spatial information that corresponds to a rotation result when an error amount calculated by the error calculation unit becomes substantially the minimum for each frequency band of the input signal.
 6. A downmixing method comprising: matrix converting to perform an matrix operation for an input signal; rotation correcting to apply a rotation to the matrix operation result at the matrix converting; extracting spatial information from a result of the rotation at the rotation correcting; calculating an error amount of the matrix operation result for the input signal by performing a matrix operation for the rotation result at the rotation correcting and the spatial information extracted at the extracting spatial information using a matrix that is inverse to the matrix used at the matrix converting; comparing a new error amount obtained by the calculating the error amount with an error amount in the past; updating the rotation result and spatial information in the past to a new rotation result and spatial information extracted at the extracting spatial information that correspond to the new error amount when the new error amount obtained at the comparing errors is less than the error amount in the past; and repeating the rotation correcting, the extracting spatial information, the calculating the error, the comparing errors and the updating while changing a rotation angle for the matrix operation result at the matrix converting.
 7. The downmixing method according to claim 6, wherein the extracting spatial information calculates, as the spatial information, a coefficient for each vector when a signal to be predicted among the matrix operation result at the matrix converting is decomposed into vectors of the rotation result at the rotation correcting.
 8. The downmixing method according to claim 6, wherein the rotation correcting determines a rotation result when the error amount calculated at the calculating the error becomes substantially the minimum for each frequency band of the input signal, and the extracting spatial information determines spatial information that corresponds to a rotation result when an error amount calculated by the calculating the error becomes substantially the minimum for each frequency band of the input signal. 